Hydrogen sulfide contained in liquids, such as petroleum, is highly corrosive against alloys. Accordingly, the alloys used in liquids containing hydrogen sulfide should have a good corrosion resistance to hydrogen sulfide. The examples of alloys which are employed in environments where they are in contact with hydrogen sulfide-containing liquids include drill pipes, pipes for flow lines from oil wells, oil country tubular goods for oil and natural gas wells, plate members for natural steam power stations, plate members for installation for desulfurization from exhaust gases, and the like. Especially, drilling of oil wells, exploitation and production of natural gas involve corrosive environments which are severe. For an index indicating the corrosive environment of hydrogen sulfide, the usual practice is to use a partial pressure of hydrogen sulfide in a gas phase. This is because the concentration of hydrogen sulfide in an aqueous solution is substantially proportional to the partial pressure of hydrogen sulfide in a gas phase, thus enabling one to simply express the degree of influence on alloys. With production pipes for oil wells, it has been frequently experienced that alloys are exposed to severe environmental conditions such as a partial pressure of hydrogen sulfide of about 10 atm., and a temperature of about 200.degree. C.
The corrosion of an alloy ascribed to the hydrogen sulfide in such an environment as mentioned above results mainly in the cracking of the alloy under stress (stress corrosion cracking). Accordingly, alloys which are to be employed in an environment containing hydrogen sulfide should have a good resistance to stress corrosion cracking.
Known alloys which are used in an environment such as of a hydrogen sulfide-containing oil well wherein a partial pressure of hydrogen sulfide is as high as approximately 10 atm., include Ni--Cr--Mo--Fe Ni-based alloys which contain Ni in amounts as great as 30-50% (Japanese Laid-open Patent Application Nos. 57-131340, 57-134544 and 57-134545).
For instance, Japanese Laid-open Patent Application No. 57-131340 proposes an alloy which comprises, aside from Ni, Cr, Mo and W, Cu and Co, if necessary, in order to improve the resistance to stress corrosion cracking.
These alloys for oil wells are so designed as to improve the corrosion resistance to hydrogen sulfide including a resistance to stress corrosion cracking. More particularly, the corrosion resistance to hydrogen sulfide is greatly influenced by the content of hydrogen sulfide present in raw oil and the temperature of the raw oil. Accordingly, when used in such an environment of hydrogen sulfide as having set out above, the alloy is so designed that a corrosion-resistant film is formed on the surfaces of the alloy. The corrosion-resistant film should have a two-layer structure including an outer layer consisting of a Ni sulfide film and an inner layer consisting of a Cr oxide film. In order to facilitate the growth of the inner layer of the Cr oxide film, at least one of Mo and W is incorporated in the alloy. The reason why the corrosion-resistant film is designed to have a double-layer structure is so that hydrogen sulfide is prevented from entering into the inner layer by means of the outer layer of the Ni sulfide film thereby preventing the breakage of the Cr oxide inner film with the hydrogen sulfide. The Cr oxide inner film is able to suppress the dissolution of the alloy and thus, acts to improve the corrosion resistance, ensuring a good corrosion resistance to hydrogen sulfide.
However, it has been confirmed that in an environment where the partial pressure of hydrogen sulfide is relatively low at a level of about 1 atm., or below and the temperature is about 150.degree. C., conventional alloys of the type set out above are not satisfactorily resistant to corrosion. The reason for this has now been found as follows: the Ni sulfide film serving as the outer layer which has the corrosion resistance to hydrogen sulfide is unlikely to grow under conditions where a partial pressure of hydrogen sulfide gas is low.
Moreover, since the known alloys contain both or either of Mo and W which are expensive, the resultant alloys become undesirably high in cost.
Accordingly, there is a strong demand for the development of high-Cr and high-Ni alloys which are excellent in corrosion resistance to hydrogen sulfide including a resistance to stress corrosion cracking when placed in an environment where the partial pressure of hydrogen sulfide is as low as about 1 atm., or below and the temperature is about 150.degree. C., and which are available inexpensively.
The invention has for its object the provision of a high-Cr and high-Ni alloy which overcomes the problems involved in the prior art, which is imparted with good corrosion resistance to hydrogen sulfide under environmental conditions of a partial pressure of hydrogen sulfide of 1 atm., or below and a temperature of about 150.degree. C., and which is low in cost.